1. Field of the Invention
The present invention relates to a telemetric process for measuring short distances, and is particularly applicable to continuous emission proximity radars which make use of correlative devices using binary logic sequences.
2. Summary of the Prior Art
A proximeter or telemetric radar for measuring short distances fitted to a flying machine enables the shortest distance between it and another flying machine to be measured. French Patents Nos. 2 286 391 and 2 580 080 describes examples of such proximeters, and an example is shown in FIG. 3, which operate briefly as follows.
As shown in FIG. 3, radar frequency generator 1 generates a microwave signal in the range generally used for radar, i.e., in the vicinity of 2 GHz. This signal is fed to an antenna 5 successively through an isolator 2, a phase modulator 3 and a duplexer 4. Isolator 2 separates radar frequency generator 1 from phase modulator 3 to avoid frequency drifts which could occur as a result of a mismatch. Phase modulator 3 provides a modulation, by a phase shift known as O-II, of the signals S1 or S2 coming from the switching threshold generator 32 through pseudo-random sequence generator 50. Duplexer 4 alternately provides a connection, of a transmission through phase modulator 3 with antenna 5, and of a reception of antenna 5 with the received signals being input into correlators 22, 24. This duplexer 4 can be of the ferrite circular type.
The received signals at antenna 5 are directed by duplexer 4 to correlators 22 and 24, where the received signals are respectively correlated with a signal identical to the transmitted modulation signal but delayed by a time T.sub.1 and T.sub.1 +.DELTA.t by shift register 30. These correlation functions and the role of comparator 26 are in particular explained in U.S. Pat. No. 4,014,021. Clock 28 controls pseudo-random sequence generator 50, and is in turn controlled by a signal output from comparator 26. Consequently, the measured distance can be deduced from clock frequency FH. This is the role of converter 19.
A continuous electromagnetic signal is modulated by a pseudo-random sequence comprising a certain number of bits generated by a feed-back shift register and delivered at a clock frequency. The modulated signal is emitted by a transmitting antenna, and the reflected signal (or echo) is detected by a receiving antenna and correlated with the modulated signal, time-delayed in a known manner. When the delay corresponds exactly to the outward and return journey of the emitted wave, for example after reflection from a target, the correlation function is at a maximum. The error voltage at the output of the correlator controls the clock frequency, which increases Progressively as the detected machine gets closer. In this way, the radar calculates from the clock frequency the distance to be measured.
It is known that this type of radar only operates within distances ranging from around 30 meters to about 2 meters. When no echo is detected, the clock frequency remains unchanged and corresponds to the predetermined distance limit, but as soon as a moving object enters the detection zone, the frequency "locks" onto the variation in distance by means of the correlation signal produced.
One can show that with such prior art proximeters, for this range of measurable distances, the machine carrying the radar must be about 300 m away from all parasitic reflecting surfaces, such as the ground.
Indeed, the periodicity of the pseudo-random sequence gives rise to a periodicity of the correlation function used for controlling the clock frequency. This periodicity involves the existence of an ambiguous distance at which one cannot distinguish between an echo reflected back from a nearby object and one situated beyond this ambiguous distance.
In general, for a sequence of n bits, the period of the correlation function is equal to (2.sup.n -1)T, namely 127 T if n equals 7, and where T is the period corresponding to the clock frequency. The period (2.sup.n -1)T corresponds to the ambiguous distance. Now, T reduces (or, alternatively, the clock frequency increases) as the measured distance decreases, because the one is tied to the other by the correlation. Thus, the ambiguous distance reduces as the detected device approaches, which can upset the measuring process by making it imprecise.
Furthermore, various spurious signals can impair the detected signal and cause an incorrect measurement. For example a parasitic echo returned by a surface (the ground, for example) very much larger than that of the device to be detected has a very large amplitude (which can be as much as 10.sup.4 times the amplitude of the echo from the device to be detected). Other interference can be caused by poor decorrelation between the transmitting and receiving antennae or by leakages at the output of the correlator.
Usually, in order to avoid having to take into account these sources of interference or permanent echos, the detection is effected in a frequency band ranging from 1 to 20 kHz, which corresponds to the frequency drift due to the expected Doppler effect for the devices to be detected. Centering the detection on the average Doppler frequency enables the signal to noise ratio of the measurement to be improved.
FIG. 1 of the drawings represents diagrammatically the shape of the frequency spectrum of a signal modulated by a sequence of n bits delivered at the clock frequency fH.
This spectrum has lines with a spacing of fH/2.sup.n -1, and for ease of representation, the number of lines shown has been reduced. The hatched zone represents the Doppler dispersion zone within which the detection is carried out. One can se that the clock frequency and the number of bits are chosen so that the first spectral line does not fall within the detection zone.
During detection, the spectral lines move apart from one another and the spectrum is widened.
It will be understood that with a fixed number of bits, filtering imposes a lower limit upon the clock frequency. The operational limitations of the clock frequency demand a considerable separation from parasitic surfaces for the range of distances to be measured (a separation of about 300 m for a range of around 30 meters).